The present invention generally relates to HDB3 code violation detectors, and more particularly to an HDB3 code violation detector which detects a code violation when demodulating a digital transmission HDB3 code into a non-return-to-zero (NRZ) code by collating the HDB3 code with respect to the demodulated NRZ code for the purpose of checking the quality of the digital line.
Generally, when systems are coupled via a PCM (Pulse Code Modulation) digital line, the HDB3 code which is a digital transmission code in conformance with the CEPT (European Conference of Postal and Telecommunication Administrations); system PCM 30 of the CCITT (International Telegraph and Telephone Consultative Committee) Recommendations G.732 is used.
The data transmitted and received between the systems is formatted, and each system reads a deterioration in the data format when the received digital transmission code is demodulated in the NRZ code, so as to detect the quality of the digital line.
In the recent CCITT Recommendations G.821, the detection as a code violation of the digital transmission code is also recommended as an important item in monitoring the quality of the digital line.
In order to add the new function of detecting the HDB3 code violation to the system which is in conformance with the CEPT system PCM 30 of the conventional CCITT Recommendations G.732, it is necessary to realize a circuit which is of a minimum scale or size and is capable of detecting the code violation with high accuracy.
FIG. 1 is a diagram for explaining an example of a conventional HDB3 code violation detection system. The correspondence of the positive and negative pulses of the HDB3 code with respect to the NRZ signal is as shown in FIG. 1, and two kinds of checks C1 and C2 are made as shown.
The first check C1 checks a mark "B" and one immediately preceding bit. On the other hand, the second check C2 checks a violation pattern "B00V" or "100V" and the alteration or combination of the positive and negative pulses, where "V" denotes a violation. In other words, the second check C2 makes a pattern check and an alternation check. In FIG. 1, the first check C1 is indicated by leftwardly descending hatchings, and the second check C2 is indicated by rightwardly descending hatchings.
However, the conventional system has the following four problems.
First, the first check C1 does not make an alternation check on the mark "B".
Second, the second check C2 does not make clear the normality of the mutual relationship between the pulse of the past mark "B" and the violation "V" of the violation pattern.
Third, no check is made for the case where no pulses are present.
Fourth, in the checks C1 and C2, no check is made as to the opposite polarity of the violation pattern and the pulse of the mark "B" or the violation "V".